Mode name groups

LU 6.2 associates each session with a set of characteristics for a type of device. This set of characteristics is called a mode name. A mode name defines characteristics such as pacing level and class of service. When transaction programs request a conversation, they usually do not specify which session to use for the conversation, but they can specify the mode name of the session. Each mode name is defined in the logon mode table, which describes the session parameters. For additional information on logon mode tables, refer to z/OS Communications Server: SNA Resource Definition Reference and z/OS Communications Server: SNA Programming.

Each mode name can be used by several sessions. In an LU type 6.2, these sessions form a group that is treated as a pool of sessions, all of which share the same mode name characteristics. An application program can control the size of a pool, but VTAM® handles the individual sessions that make up the pool. You cannot specify pool size for non-LU 6.2 sessions.

LUs can define a number of different mode name groups for sessions with another LU. For example, an LU might have a FILESERV mode name defined for sessions with a partner LU used by file-server programs. FILESERV denotes sessions with a large request unit (RU) size, which aids bulk transmission of data. This LU also might have other mode names, such as INTERACT, defined for sessions with the same LU used for database queries.

By using different mode groups, you can avoid the possibility of database queries getting backed up behind mass file-server requests. To determine whether different mode name groups are beneficial, consider the following factors:

Importance of the data exchange
Nature of the data exchange
Desired response time

As part of its support for LU 6.2, VTAM maintains a data structure for the application program that:

Lists possible partner LUs
Determines valid names for the partner LU
Associates valid mode names defined for each partner LU
Contains information about the mode names

This data structure is the LU-mode table. It is unique in VTAM to LU 6.2 support. For more information on the LU-mode table, see Data structures.

Some SNA-defined modes are used for special purposes. For example, the SNASVCMG mode is used by VTAM to exchange control information. Application programs should not normally use SNASVCMG sessions for a conversation. The CPSVCMG mode is defined but it is also reserved.